Scleractinian-specific eDNA metabarcoding (eDNA-M), based on amplification of mitochondrial 12S rDNA, has become an effective tool for comprehensive monitoring of zooxanthellate scleractinians at the generic level, but several questions remain about its efficiency. Taking advantage of coral culturing tanks at Okinawa Churaumi Aquarium, Japan, we examined the robustness of this metabarcoding technique. Our eDNA-M system can theoretically identify 53 scleractinian genera. Without knowing genera and numbers of colonies in a large, indoor exhibition tank, eDNA-M identified 29 genera in tank effluent and one genus that requires further bioinformatic improvement for detection. Later, aquarium staff informed us that the tank contains 37 genera. Colony numbers in the tank were also provided. Tank population data revealed that mitochondrial 12S rDNA of the seven missing genera had not been sequenced yet. Those seven taxa were sequenced thereafter, with the result that at the end, 36 of the 37 genera were identified. Even three genera with only one colony in the tank were detected. Surveys of outdoor culture tanks yielded comparable results, with additional genera that may have been detected due to contamination in incurrent natural seawater. We conclude that this scleractinian-specific eDNA metabarcoding is robust for coral monitoring, although further mitochondrial 12S rDNA sequences are needed for additional scleractinian genera.

Symbiosis with chemosynthetic bacteria is a biological phenomenon that has enabled animals to adapt to deep-sea environments. The deep-sea vent mussel Bathymodiolus septemdierum harbors sulfur-oxidizing symbionts in its gills, which serve as its main source of nutrients. However, the establishment and maintenance of this symbiosis process remain poorly understood, partly because culturing deep-sea mussels and their symbionts is difficult and experimental studies are rare. In the present study, we aimed to establish a rearing method for B. septemdierum and investigate the effects of sodium sulfide (Na2S) addition and gas concentration control in rearing tanks. Three tanks were prepared: a normal tank (tank N), one with Na2S (tank S), and one with Na2S and control dissolved oxygen (tank SO). Mussels were maintained for more than 1000 days, and host survival, gill histology, and the abundance of symbionts were investigated. In tanks N and S, symbionts were depleted within a short period, whereas in tank SO, gill morphology and symbiont abundance were maintained at levels comparable to those of freshly collected individuals, even after 1000 days. However, the survival rate of the host did not correspond to symbiont maintenance. Phylogenetic analysis revealed that the symbiont RuBisCO exists in Form II, which is generally adapted to low-O2 and high-CO2 environments. These results suggest that the long-term maintenance of B. septemdierum symbionts is possible by adding Na2S and controlling the dissolved gas concentration. Further improvements in rearing methods could contribute to a deeper understanding of host-symbiont interactions in this unique deep-sea organism.

Scientific names, especially epithets (specific names in the zoological nomenclature), are derived from various factors, not only species characteristics but also cultural backgrounds, such as the names of people. They reflect how species were perceived at the time. However, several ethical issues have been raised, such as naming species after criminals and gender imbalance in eponyms (epithets named after people). Previous research has been conducted through thorough literature reviews with random sampling, which requires significant time and effort. In this study, the accuracy of the automated labeling using a large language model (LLM) was assessed, and the temporal etymological trends of 2705 species of phytophagous arthropods were investigated. LLM-based classification achieved F1 scores above 75% and accuracy above 90% in Morphology, Host, Geography, and People. However, Ecology & Behavior and Other exhibited accuracy issues. Analyses using the generalized additive model (GAM) revealed shifting naming trends, with a decrease in Morphology and an increase in Geography and People, consistent with previous research on spiders. This study demonstrates the effectiveness of LLM-based classification for epithets and provides a new perspective on the social and scientific debates surrounding scientific names based on etymological trends.

The mechanisms underlying sexual differentiation in viviparous chondrichthyans remain poorly understood, particularly regarding the impact of the maternal environment on early gonadal differentiation. In this study, we examined gonadal and reproductive duct differentiation in red stingray (Hemitrygon akajei) embryos before and after clasper formation through detailed histological analyses. Histological observations revealed that early embryos (stage 3, before clasper formation) exhibited undifferentiated gonads with scattered primordial germ cells, whereas later embryos (stage 4, during clasper development) exhibited clear sexual dimorphism. Female gonads contained numerous germ cells localized in the dorsal region, whereas male gonads contained fewer scattered germ cells. Both the Müllerian and Wolffian ducts were present in both sexes, with the Wolffian ducts emerging from the Müllerian ducts in a segmental pattern. Stage 4 female embryos exhibited a selective enlargement of the right Müllerian duct, establishing reproductive tract asymmetry. The sex ratio of red stingray embryos was 1:1. These findings provide the first detailed histological characterization of gonadal differentiation and reproductive duct development in a viviparous chondrichthyan and establish an essential foundation for future studies on sexual development in these evolutionarily significant vertebrates.

Copy number variation (CNV) in gene loci in animals can be driven by adaption to the environment. The relationship between CNV in genes for amylase (AMY), which hydrolyzes starch, and dietary adaptation has been well studied. Copy number (CN) of AMY is higher in human populations with high-starch diets, compared with those with low-starch diets. Although CNV in AMY has been reported in humans and some domestic animals, there have been few studies of animals in the wild. The brown bear (Ursus arctos), widespread in the Northern Hemisphere, shows large dietary variation among individuals and groups. Brown bear population genetic structures are associated with the dispersal history due to climate change over the past few tens of thousands of years, and this together with dietary variation should have led to CNV in AMY. In this study, we investigated CNV in AMY in brown bears worldwide by using whole genome sequencing data. We detected AMY CNV among regional groups. AMY CN was similar among brown bears in geographically proximate populations, such as between Hokkaido (Japan) and East Asia, and between the North America mainland and the ABC islands. CNs were smaller in bears from East Asia, including Hokkaido, compared to those from other regions across brown bears' distribution. Our results suggest that CNs of AMY reflect the population demographic history of brown bears after the Last Glacial Maximum.

Three non-encysted digenean metacercariae were found parasitic in the mesoglea of two of 13 individuals of the staurozoan cnidarian Haliclystus tenuis Kishinouye, 1910 from Yoichi, Hokkaido, Japan. The metacercariae comprised two morphospecies (one oval, the other elongate), for which the gross morphology is described. Partial sequences of the 18S and 28S rRNA genes and the internal transcribed spacer 2 region were determined for both species, and a partial sequence of the cytochrome c oxidase subunit I gene was determined for the oval species. Molecular phylogenetic analyses revealed both digeneans to be in the family Lepocreadiidae, a taxon not previously reported from staurozoans. The oval species was a member of the "Diploproctodaeum Clade." The elongate species was identified as Prodistomum orientale (Layman, 1930), known to use marine fishes in the genus Scomber Linnaeus, 1758 as definitive hosts. Second intermediate hosts of P. orientale known around Australia include five hydrozoan, one scyphozoan (Cnidaria), and one ctenophoran species. This is the first information on a second intermediate host from Japan; however, we cannot rule out the possibility that staurozoans may also be dead-end hosts. Including an opecoelid species previously reported, three digenean species in two families are now known from a single staurozoan species in the small region comprising the northwestern coast of Hokkaido. Future surveys for parasites in other staurozoan species and regions will likely detect additional digeneans and other parasite groups utilizing staurozoans.

Understanding the growth of organisms is of fundamental importance in achieving the sustainable use of bioresources and/or the conservation of ecosystems. Hagfishes are oceanic scavengers that typically inhabit deep water and play an indispensable role in marine nutrient cycling. However, as the conventional methods for age determination are not applicable, the growth and lifespan of hagfishes are completely unknown. In the present study, the growth and feeding of the inshore hagfish (Eptatretus burgeri) in the laboratory were continuously monitored for an unprecedentedly long period (more than 1.4 years), with feeding once every 4-5 weeks. The animals displayed either positive/negative/zero growth across the experimental period. Eptatretus burgeri typically takes 8-10 days to digest food, and excretes all the gut contents at once as huge feces. Based on the growth, feeding and excretion data, it seems plausible that the wild populations feed at less than 2-week intervals. In addition, by adopting a recently developed technique to estimate fish age from the relative growth data, the age-size relationship in E. burgeri was explored. The generated growth curve predicted that the gonads start to develop only 4 years after hatching at the earliest. Eptatretus burgeri can live nearly 50 years, and the animals targeted in commercial hagfishery are mainly 6-9 years old. Moreover, additional insights into the oviposition cycle were obtained. The present study provides essential information for understanding the ecology of hagfishes and consequently highlights the putative vulnerability of wild hagfish populations to fishing and environmental stresses.

In this study, we obtain and analyze the first transcriptome data for the family Thamnocephalidae, targeting Branchinella kugenumaensis (Ishikawa, 1895), an anostracan fairy shrimp primarily found in Japan and Far Eastern Eurasia. We collected a male specimen from a rice paddy in Tochigi, Japan, and performed RNA-seq analysis, obtaining an assembled transcriptome comprising a 15,157 nucleotide sequence with an 80.75% completeness score. Because the relationships among the families within Anostraca remain unclear, using both mitochondrial and nuclear genes, we constructed phylogenetic trees employing maximum likelihood and Bayesian methods. Our results indicate that Artemiidae formed a cluster with Thamnocephalidae, and together they formed a sister group to the other Anostraca. We also demonstrated that a large number of parallel or back substitutions can hinder the estimation of phylogenetic relationships. This study not only enhances our understanding of anostracan phylogeny but also highlights the significance of transcriptome data in evolutionary biology.

The heteronemertean genus Lineus Sowerby, 1805, currently accommodating about 100 species, is known to be non-monophyletic and is in a desperate need of taxonomic revision. In this article, we focus on a group of species including L. caputornatus Takakura, 1898, L. grubei (Hubrecht, 1879) auct., L. pictifrons Coe, 1904, and L. subcingulatus Takakura, 1898. These four morphologically similar species formed a clade to the exclusion of the type species, L. longissimus (Gunnerus, 1770), on a molecular phylogeny based on partial sequences of the 16S rRNA, 18S rRNA, 28S rRNA, cytochrome c oxidase subunit I, and histone H3 genes. For this clade we propose the name Bilucernus gen. nov. We redescribe L. grubei auct. from Japan as Bilucernus takakurai sp. nov. Bilucernus clade was nested within a larger clade that also contained members of Kulikovia Chernyshev, Polyakova, Turanov, and Kajihara, 2017, Lineopselloides Gibson, 1990, Siphonenteron Renier in Meneghini, 1847, and Tenuilineus Riser, 1993. We observed that body fragments from the intestinal region regenerated a head in B. caputornatus comb. nov. and B. subcingulatus comb. nov., but did not in B. takakurai sp. nov. The type specimen of Lineus bipunctatus Takakura, 1898 was most likely a regenerating individual of B. caputornatus comb. nov. We synonymize L. bipunctatus with L. caputornatus, giving nomenclatural priority to the latter. In addition, Lineus albifrons Coe, 1934 is herein also synonymized with L. caputornatus. We speculate that the unusually wide geographic distribution of B. caputornatus comb. nov. is related to its ability to regenerate head from fragments of body.

Resting cyst formation is a strategic aspect of the life cycle of some eukaryotes such as protists, and particularly ciliates, that enables adaptation to unfavorable environmental conditions. The formation of resting cysts involves large scale morphological and physiological changes that provide tolerance of extreme environmental stresses. The resting cyst shows suppression of normal features of life such as eating, moving, proliferation, and even mitochondrial metabolic activity, and appears lifeless. This review discusses resting cyst formation in the ciliates Colpoda as a representative model of cyst-forming organisms, and focusses on morphogenesis, molecular events, tolerances, and metabolic activities in resting cysts.